In recent years, optical disks have become a standard medium for the distribution of digital data. Their relatively low cost and high storage density have led to their wide-spread use with personal computers. At present, almost every new personal computer system includes an optical disk drive capable of retrieving data from read-only optical disks (CD-ROM), and optical disks are rapidly becoming the distribution medium of choice for software publishers. With the cost of devices for reading and writing optical disks decreasing, and the amount of information which can be stored on a disk increasing, it seems likely that the popularity of optical disks for storing digital data will continue into the foreseeable future.
A typical optical "compact disk" (CD) includes a transparent plastic substrate having data encoded in pits that are impressed into the surface of the substrate along a spiral track. A metalization layer deposited on the pitted substrate provides a reflective surface, and a protective transparent layer is then deposited on the metalization layer. To read the data, an optical disk drive uses an optical pickup assembly which reflects a reading beam of coherent light off of the metalization layer of the disk and uses a detector to sense the intensity of the reflected light.
As the disk is rotated, pits along a data track sequentially pass under the spot projected onto the disk by the reading beam. The presence of a pit in the data track causes destructive interference to occur between light reflecting from the pit and light reflecting from the area surrounding the pit. The intensity of the reflected light is thus modulated by the pattern of data pits in the disk substrate.
The modulated, reflected light is directed to a detector that develops electronic signals corresponding to the intensity of the reflected light. These electronic signals are then demodulated by processing circuitry to recover the digital information stored on the optical disk. Further details regarding the construction and use of optical disk drives can be found in Compact Disc Technology, Nakajima, H. and Ogawa, H., translated by Aschmann, C., published by Ohmsha, Ltd., Japan (1992), and The Compact Disc Handbook, Pohlmann, K., 2nd ed., A-R Editions, 1992.
The increased availability of CD-ROM products, coupled with the availability of increasingly faster microprocessors, has created a demand for ever faster optical disk drives. As a result, optical disk drives capable of rotating the disk at multiples of the rotation speed of a standard single speed drive are becoming available. Drives which rotate the disk at speeds of 8 times and up to 12 times the speed of a standard single speed drive are currently available. In an 8X (eight times single speed) drive, for example, the disk is rotated at speeds up to 4800 rpm when reading the innermost data track, as compared to approximately 600 rpm for a single speed drive. The ability to achieve even greater speeds using this method may soon be limited by the ability to provide low-cost, easily manufacturable drives. The use of greater disk rotational speeds requires optical drive designs which are more sophisticated, and require tighter manufacturing tolerances. This results in drives which are more expensive to design and produce than previous optical disk drives.
A cost effective alternative to increasing the disk rotational speed to provide faster optical disk drives is to read multiple data tracks simultaneously, as described in commonly assigned U.S. Pat. No. 5,426,623 to Alon et al., the entirety of which is incorporated herein by reference. In accordance with the methods and apparatus provided therein, for example, ten adjacent data tracks may be read simultaneously. Thus, even if the disk is rotated at only four times the standard speed (i.e., a 4X drive is used), the capability to read ten tracks simultaneously provides the equivalent of a 40X drive.
It should be noted that as used herein, a data track is a portion of the spiral data track of a typical optical compact disk which follows the spiral for one rotation of the disk. Thus, a drive capable of reading multiple data tracks simultaneously reads multiple such portions of the spiral data track at once. For optical disks having concentric circular tracks, a data track would refer to one such circular track. For disks having multiple concentric spiral tracks, such as those described in commonly assigned, copending U.S. patent application Ser. No. 08/885,425, a data track would refer to one of the concentric spiral tracks.
One way in which a drive capable of reading multiple data tracks simultaneously may be implemented is through use of multiple beams, arranged so that each beam illuminates a single data track on the disk. U.S. Pat. No. 5,144,616 to Yasukawa et al. shows a system in which multiple laser diode emitters are used to provide multiple beams. Other methods may also be used to provide multiple beams. U.S. Pat. No. 4,459,690 to Corsover, for example, describes a multi-beam system in which an illumination beam generated by a single laser source is split into multiple beams using an acousto-optic device that dithers the beam in a direction normal to the track direction.
The beams in a multi-beam optical pickup may also be provided by using a diffractive element to split a single beam into multiple beams. This technique is used to generate the beams in a three-beam tracking system, as shown in The Compact Disc Handbook, Pohlmann, K., 2nd ed., A-R Editions, 1992, pp. 108-115. In commonly assigned U.S. patent application Ser. No. 08/911,815, filed concurrently herewith, a diffractive element is used to split an illumination beam into a plurality of co-linear reading beams. Through careful design, it is possible to produce a diffractive element capable of generating multiple beams having the proper spacing to align with-the data tracks of an optical disk. A holographic element, such as the one described in U.S. Pat. No. 5,272,550, to Dickson et al. could also be used to achieve this effect.
In addition to being aligned with the data tracks, the beams in a multi-beam optical pickup must be maintained at specified distances from each other to avoid crosstalk between data tracks, and to properly align the beams with detectors. These distances are determined by the spacing of the tracks on the optical disk, the magnification of the optics, and the size and spacing of the detectors which are used to read the information. The necessary spacing between beams can be decreased by increasing the magnification of the optics or by decreasing the size and spacing of the detectors. Increasing the magnification of the optics reduces the optical efficiency of the system, and reducing the size of the detectors makes them less effective, and more costly to manufacture. The spacing of the beams in a multi-beam system represents a tradeoff between these factors. If the size, sensitivity, and cost of photodetectors improve in the future, it may be possible to reduce the spacing between the beams.
The multi-beam system described in the above-referenced U.S. patent application Ser. No. 08/911,815 (ZRI-011) has its plurality of reading beams arranged in a single row. This row is typically angled with respect to the radial direction of the disk, often by an angle greater than 85 degrees, to maintain the needed distances between spots projected onto the surface of the disk as determined by the beam spacing. As more beams are used to read more data tracks simultaneously, the row of beams becomes longer, taking up a larger portion of the area which is picked up by the optics (hereinafter referred to as the field of view). Points near the outer extremes of the field of view of an optical system suffer increased optical aberrations compared to points near the center of the field of view. The outermost beams may also suffer from vignetting, causing less of their energy to reach the detectors. The size of the field of view, and the degree of aberration and vignetting at the edges of the field of view place a practical upper limit on the number of data tracks which may be read simultaneously using multiple beams. Increasing the field of view of the optics to reduce the aberration and vignetting increases the cost, and reduces the optical efficiency of the system.
It would therefore be desirable to reduce the field of view required to pick up all of the beams in a multi-beam optical disk drive system, while maintaining or increasing the number of beams, and maintaining specified minimum distances between the beams. Achieving this goal would reduce the degree of optical aberration and vignetting affecting the outer beams, increase optical efficiency and performance, and enable optical disk drives to be manufactured that can simultaneously read a greater number of data tracks than previously known multi-beam systems.